Daily Current Series from the Deep Eastern Gotland Basin ( ) E. Hagen and G. Plüschke

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1 Daily Current Series from the Deep Eastern Gotland Basin ( ) E. Hagen and G. Plüschke 1. Introduction This report presents data of current measurements initiated by three research projects of the Institute for Baltic Sea Research Warnemuende (IOW) to study low-frequency fluctuations in deep currents of the Eastern Gotland Basin. The first of them was the interdisciplinary Gotland Basin Experiment (GOBEX, ), cf. Hagen (1996), Zülicke & Hagen (1997), and Emeis & Struck (1998). The second one focused on hydrographic phenomena expected on spatial scales with a diameter of several kilometres. It was denoted Meso-Scale Dynamics, (MESODYN, ) and is described in more detail by Reißmann (1999), Hagen & Feistel (2001), and Reißmann (2005). Related field campaigns focused on spatiotemporal changes in thermohaline properties of the deep mass field and associated circulation patterns in the Arkona Basin, the Bornholm Basin, the Stolpe Furrow, and the Eastern Gotland Basin (EGB). Unfortunately, intense fishing activities prevented long-term records of currents in the three shallower basins. Consequently, such measurements were carried out preferentially in deep layers of the EGB although the existence of hydrogen sulphide required special mooring techniques. Logistic frame conditions limited all long-term current records at only one position located above the north-eastern topographic flank of the deep EGB denoted NE-1 and NE in Fig.1. These activities could be continued during the third project denoted Deep Rim Currents in the Eastern Gotland Basin (RAGO, ) residing in its synthesis-stage. Other long-term records of deep currents came from biological studies carried out in the central EGB between 2004 and One Aanderaa recording current meter (RCM) was mounted few metres beneath a sediment trap moored in vicinity of the central HELCOM monitoring station 271 (57 20 N, E) where the overall water depth is about 245 m, Reißmann (1999). Records from this position are labelled CENTRAL in this report, Fig.1. Due to different recovering/ deployment episodes of related sediment traps and slight changes in the overall water depth around the target depth, the actual measuring horizon of this RCM fluctuated by few metres. However, such little discrepancies have been neglected and these measurements are considered to be 60 m above the bottom. All other sub-surface strings were equipped with 3-5 RCMs at positions echo-sounded to be in the range between 200 and 240 m, depth, Fig.1. The only exception was the string deployed at 70 m depth above the southern flank of the so-called Klints Bank during four days in April For completeness, these series have been included on the base of hourly averages. 1

2 Fig.1 Study area in the central Eastern Gotland Basin (upper panel) and its bathymetric contours (m) showing shallow water areas of the Klints Bank as well as the so-called Gotland Deep in the centre; positions of moored sub-surface strings were equipped with Aanderaa recording current meters during several field campaigns carried out between1993 and 2008 whereby assigned projects and used string abbreviations are compiled in Table 1. The schematic of exposed sub-surface strings, which were equipped with three RCMs at the mooring position denoted NE-1 and NE in Fig.1, is shown exemplary in Fig.2. Their position was seen to be suitable to study low-frequency changes in deep rim currents. 2

3 Fig.2 Schematic of sub-surface strings equipped with three recording current meters (RCM) deployed at the positions NE-1/ NE shown in Fig.1. Project Identifier Start End Days Strings Levels GOBEX GB-93(1/2/3) / 2/ 3 GB-94(1/2/3) / 3/ 3 GB MESODYN SW NE KB C-00(1/2/3) /9/10 3 1/ 2/ 4 CENTRAL Z Z RAGO SW SE Long-Term NE NE-P NE-P NE-P NE-P NE-L Table1. Project abbreviation, mooring identifier, starting and ending day of resulting series, overall length, the number of deployed strings, and the number of associated measuring levels at positions labelled in Fig.1. 3

4 2. Data Base Mechanically operating current meter types RCM-7/8 exhibit a resolution of ±1 cm/s while that of the electronic type RCM-9 is better by the factor of about three (±0.3 cm/s). The used sampling interval (SI) resulted from particular project-tasks and varied between 1 minute (short-term records) and 1 hour (long-term records). Resulting time-series of current speed and current direction were decomposed into those of the zonal component (u, positive to the east) and the meridional component (v, positive to the north). All records, which were based on sampling rates shorter than an hour, were transformed into hourly series. These hourly series were used to compute the daily averages pointing to the UTC-noonday. Missing data were completed by linear interpolation between neighbouring values. Such gaps of 3-4 hours were caused by the time needed for recovering and deploying consecutive strings. Resulting recording lengths are shown in Fig.3. Fig.3 Lengths of daily current series recorded by Aanderaa recording current meters (RCM-7/8/9) within deep layers of the Eastern Gotland Basin at positions shown in Fig.1; long- term series only result from the position NE equipped with three RCMs mounted at 5, 20, and 50 m above the sea bed (220/ 224 m) as shown schematically in Fig.2; complete measuring periods for all three horizons are labelled P2, P3, and P4 while P1 reflects a somewhat longer series from two RCMs deployed 20 and 50 m above the bottom; the longest continuous time series (denoted NE-L in Table 1) covers 2253 days and results from the 20m-horizon. 4

5 In the following, the presentation of each measuring campaign starts with a map showing bathymetric contours and the corresponding string position. The accompanying table explains the mooring identifier (project/ campaign/ string abbreviation), the position ( N, E), the water depth (m), the sampling interval (SI, min=minute, h=hour), and the first and the last complete measuring day. Appropriate overall statistics include the recording length (N), the arithmetic mean, the standard deviation (STD), the skewness and the kurtosis (vanishing for a Gaussian frequency distribution), the maximum (Max.), the minimum (Min.), and the total range of fluctuations (Max.-Min.), cf. Wilks (1995). Daily series of both current components are plotted versus the time to elucidate roughly the variability in zonal (u) and meridional currents (v) while progressive vector diagrams (PVD) characterize the time history of currents at different measuring horizons labelled by their distance above the sea bed. Different sub-series of the presented data sets were used in previously published studies, frequently in context to corresponding temperature records and snapshot surveys of quasi-synoptic hydrographic surveys, cf. Mittelstaedt (1996), Stips (1996), Zülicke et al (1998), Feistel et al (2003a), Feistel et al (2003b), Hagen & Feistel (2004), Feistel et al (2004), Hagen (2005), Reissmann (2006), Feistel et al (2006), Hagen & Feistel (2007), and Wieczorek et al (2008). Finally, the longest continuous record of deep currents (NE-L, 2253 days, 20 m above the sea bed) is presented by several supplementary graphs to give a certain impression about characteristic temporal scales at this position. All daily series of zonal/meridional current components, which are tabulated in Table 1, have been attached in the appendix by EXEL-spreadsheets. Acknowledgements We like to express our gratitude to all scientists in charge of the IOW-HELCOM-Baltic- Monitoring-Programme (COMBINE) conducted on behalf of the Bundesamt für Seeschifffahrt und Hydrographie (BSH). They always supported the mooring activities carried out during many years at the NE-position. Dr. Falk Pollehne and U. Hehl kindly replenished data from the campaign CENTRAL. The project GOBEX was partially funded by the Kultusministerium Mecklenburg-Vorpommern (contract 0710, MG04, ) and records of the measuring campaign GB-93 were kindly provided by Dr. E. Mittelstaedt (BSH- Hamburg). Activities of the project MESODYN were organizationally accompanied by the Federal Ministry of Education, Scientific Research and Technology of Germany (BMBF), while the still running project RAGO was broadly financed by the Deutsche Forschungsgemeinschaft (DFG, No HA 1900/3-2). 5

6 3. References Emeis, K. C., Struck, U., Gotland Basin Experiment (GOBEX) Status Report on Investigations concerning Benthic Processes, Sediment Formation and Accumulation. IOW Marine Science Reports, 34, Feistel, R., Nausch, G., Hagen, E., 2003a. The Baltic inflow of autumn IOW Marine Science Reports, 54, Feistel, R., Nausch, G., Matthäus, W., Hagen, E., 2003b. Temporal and spatial evolution of the Baltic deep water renewal in spring Oceanologia, 45, Feistel, R., Nausch, G., Hagen, E., Unusual Baltic inflow activity in and varying deep-water properties. Oceanologia, 48, Feistel, R., Nausch, G., Matthäus, W., Lysiak-Pastuszak, E., Seifert, T., Sehested Hansen, I., Mohrholz, V., Krüger, S., Buch, E., Hagen, E., Background data to the exceptionally warm inflow into the Baltic Sea in late summer of IOW Marine Science Reports, 58, Hagen, E., 1996 (editor). GOBEX-Summary Report. IOW Marine Science Reports, 19, Hagen, E., Beobachtungen fluktuierender Randströme im 'Östlichen Gotlandbecken'der Ostsee. Meeresumwelt-Symposium 2004, BSH Hamburg/ Rostock, Hagen, E., Feistel, R., Spreading of Baltic deep water: a case study for the winter IOW Marine Science Reports, 45, Hagen, E., Feistel, R., Observations of low-frequency current fluctuations in deep water of the Eastern Gotland Basin/ Baltic Sea. Journal of Geophysical Research, 109, doi: /2003JC Hagen, E., Feistel, R., Synoptic changes in the deep rim current during stagnant hydrographic conditions in the Eastern Gotland Basin, Baltic Sea. Oceanologia, 49, Mittelstaedt, E., The subsurface circulation in the Gotland Deep. IOW Marine Science Reports, 19, Reißmann, J., Bathymetry of four deep Baltic basins. Deutsche Hydrographische Zeitschrift, 51, Reißmann, J., An algorithm to detect isolated anomalies in three-dimensional stratified data fields with an application to density fields from four deep basins of the Baltic Sea. Journal of Geophysical Research, 110, doi: /2005jc Reissmann, J., On the representation of regional characteristics by hydrographic measurements at central stations in four deep basins of the Baltic Sea. Ocean Science, 2, Stips, A., First investigations of the near surface turbulence structure and energy dissipation caused by wind mixing in the Baltic Sea. IOW Marine Science Reports, 19, Wieczorek, G., Hagen, E., Umlauf, L., Eastern Gotland Basin case study of thermal variability in the wake of deep water intrusions. Journal of Marine Systems, accepted Wilks, D. S., Statistical methods in the atmospheric science: an introduction. San Diego, Academic Press, 464 pp. Zülicke, C., Hagen, E., GOBEX Report Hydrographic Data at IOW. IOW Marine Science Reports, 21, Zülicke, C., Hagen, E., Stips, A., Dissipation and mixing in a coastal jet: a Baltic Sea case study. Aquatic Sciences, 60,

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59 Frequency Distribution/ Lag Correlation 0,4 u vs.v 0,2 Correlation 0,0-0,2-0, Lag (d) 59

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61 Autocorrelation 61

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65 60-Days Running Statistics (*): u* 65

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70 Appendix: Data Link: 70